1. Technical Field of the Invention
The present invention relates generally to a gas sensor which is to be installed in an exhaust pipe of automotive engines to measure the concentration of gas such O2, NOx, or CO, and more particularly to an improved structure of such a gas sensor which is designed to minimize thermal damage to a joint between a housing and a sensor element protective cover placed in a high-temperature atmosphere of exhaust gasses.
2. Background Art
Gas sensors are know which are installed in an exhaust pipe of automotive vehicles for use in air-fuel ratio control of engines.
FIGS. 14 and 15 show one example of such a type of gas sensor which is taught in Japanese Patent First Publication No. 2000-241380 (corresponding to U.S. Pat. No. 6,214,186 B1, issued Apr. 10, 2001, assigned to the same assignee as that of this application).
The gas sensor 9 includes a gas sensor element 19 working to measure the concentration of a given component contained in gasses, a housing 90 within which the gas sensor element 19 is disposed through an insulation porcelain 12, a gas sensor element protective cover assembly 91 joined to a head end of the housing 90, and an atmosphere side cover 2 joined to a base end of the housing 90.
The gas sensor element protective cover assembly 91 is of a double-walled structure consisting of an outer cover 92 and an inner cover 93. The outer and inner covers 92 and 93, as clearly shown in FIG. 15, have flanges 920 and 930 extending outward. The housing 90 has a cover installation groove 903 formed in the head end thereof and an annular extension 902 extending outside a base end surface 901 of the housing 90. Installation of the gas sensor element protective cover assembly 91 on the housing 90 is achieved by placing the flanges 920 and 930 of the outer and inner covers 92 and 93 within the cover installation groove 903 and bending the extension 902 inward to retain the flanges 920 and 930 within the cover installation groove 903.
The housing 90 has formed on an outer wall, as denoted at B in FIG. 15, thereof an external thread 904 which is engageble with an internal thread cut in an inner wall of a sensor mount hole formed in an exhaust pipe of the automotive engine. The housing 90 also has, as shown in FIG. 14, a tapered shoulder 906 formed on an inner side wall 905 which bears the insulation porcelain 12. Specifically, the housing 90 is complex in structure and must be made of material easy to machine.
When the gas sensor 9 is installed in the exhaust pipe of the automotive engine, a lower portion of the gas sensor 9 below a broken line M in FIG. 14 is exposed to the exhaust gasses, so that the gas sensor element protective cover assembly 91 is exposed to intense heat. The gas sensor element protective cover assembly 91, thus, needs to be made of heat resisting material.
Most typical heat resisting materials are difficult to machine. It is, thus, difficult to make the housing 90 with the same heat resisting material as that of the protective gas sensor element protective cover assembly 91. In a case where the housing 90 is made of material different from that of the gas sensor element protective cover assembly 91, they will have coefficients of thermal expansion different from each other, which facilitates loosening of a joint of the gas sensor element protective cover assembly 91 to the housing 90 or dislodgement of the gas sensor element protective cover assembly 91 from the housing 90.